Apparatus for controlling solid particle flow in an evaporator

ABSTRACT

An apparatus for use in an evaporator for selectively controlling the flow of solid particles within the evaporator. The apparatus includes a separation chamber having a plurality of apertures, each of which has a critical size. In the separation chamber, solid particles which are larger than a critical size are separated from the remainder of the solution. The separated particles exit the separation chamber via a bypass conduit for circulation directly to the sump of the evaporator for recombining with the separated liquid portion which passed through the separation chamber and was subsequently concentrated in the heat exchange tubes. The apparatus prevents solid particles from clogging the system and allows for continuous circulation of the solution and solid particles.

TECHNICAL FIELD

The present invention relates in general to an apparatus for selectivelycontrolling the flow of solid particles in evaporators having verticalheat exchange tubes, including falling film evaporators.

BACKGROUND ART

A wide range of industries employ evaporators for concentratingsolutions and slurries, including oil refining, synthetic fuelproduction, food processing, herbicide and pesticide production,electric generating stations, primary metal refining, pharmaceuticalproduction, and pulp and paper manufacture. The evaporators may be usedto increase the concentration of a fluid component, and/or tocrystallize a solute component. Various types of evaporators areavailable and are well known in the prior art. An example is a vertical,tube-in-shell, falling film evaporator. In such evaporators, a solutionor slurry is circulated repeatedly through heat exchange tubes. As thesolution or slurry passes through the heat exchange tubes, the solvent,i.e., water or organic solvent, is gradually evaporated, leaving a moreconcentrated solution or slurry, and often causing solute components toprecipitate.

Evaporators typically include various chambers, apertures, and tubesthrough which the solution or slurry must pass repeatedly as it isconcentrated. Care must be taken to provide for efficient flow of solidparticles as well as liquid. Slurries contain solid particles evenduring initial stages of concentration, while solutions contain solutes,i.e., salts, which may precipitate out of solution during concentration.These solid particles can cause clogging in various parts of theevaporator.

Deposits may occur along the walls or other surfaces of the evaporator.Deposits may break off in the form of chips or flakes, which can causeclogging of the evaporator and interrupt the flow of the solution orslurry to be concentrated. If the system is clogged and circulationcannot proceed efficiently, the system must be shut down to allowoperators to clear and clean the blockage.

Deposits occur while the evaporator is in operation and also while theevaporator is shut down for cleaning; as the walls of the evaporatorsystem are allowed to dry out during the cleaning operation, significantamounts of deposits can form. When the system is restarted andcirculation is restored, the new deposits often flake off and reclog thesystem.

The clogging problem is most serious when chips lodge themselves withinsmaller apertures, such as the inlet orifices of the fluid distributorsmounted at the top of heat exchange tubes. One such distributor isdescribed in U.S. Pat. No. 4,248,296. Chips interfere with normal flowof the brine, and can effectively remove entire heat exchange tubes fromoperation.

Various attempts have been made to remove the chips or flakes byfiltration. For example, screens or strainers have been fitted insidethe evaporator system. However, such screens must be cleanedperiodically, and in order to do so, the evaporator system must be shutdown, resulting in lost time, increased costs of operation, and furtherformation of deposits. In addition, when the evaporator is restarted,additional chips and flakes may form downstream of the screen orstrainer, and reclog the system.

It is, therefore, desirable to design an apparatus which will facilitateseparation of solid particles from liquid, and which will not requirethe system to be shut down for cleaning of the separation apparatus.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to separate solid particlesfrom liquid in a solution or slurry undergoing concentration in anevaporator system.

It is a further object of the present invention to provide forcontinuous separation of solid particles from liquid within anevaporator without the need for periodic shutting down of the system toclean the separation apparatus.

It is another object of the present invention to provide a bypass routethrough which solid particles can circulate through the system, ratherthan being deposited at the site of the separation apparatus.

It is still a further object of the present invention to provide anapparatus which enhances the turbulent flow of solid particles withinthe liquid so as to facilitate breaking up of solid particles intosmaller units.

These and other objects will become apparent to those skilled in the artfrom the description of the invention which follows.

The present invention discloses a solid particle flow apparatus for usein an evaporator for concentrating slurries or solutions containingsolid particles and liquid. Such evaporators may be falling filmevaporators, and the like. They include a plurality of substantiallyvertical heat exchange tubes, a lower reservoir positioned below theheat exchange tubes, a second reservoir positioned above the heatexchange tubes, and recirculation conduit for recirculating slurry orsolution from the lower reservoir to the upper reservoir.

The solid particle flow apparatus of the present invention includes aseparation chamber which has an inlet and an outlet. The inlet ispositionable to receive slurry or solution from the recirculationconduit. The outlet is connected to the bypass conduit, described below.The outlet may be positioned at substantially the lowest portion of theseparation chamber to encourage travel of the separated solid particlestoward the outlet under the influence of gravity.

The separation chamber also has a plurality of apertures which are sizedto separate solid particles from liquid. The size of each aperture is a"critical" size--a predetermined size above which the solid particlescause unacceptable clogging of various parts of the evaporator. Solidparticles larger than the critical size are preferably separated fromthe liquid to prevent clogging downstream. The chamber aperturescommunicate a substantial portion of the liquid to the upper reservoirexterior of the separation chamber for passage through the heat exchangetubes to the lower reservoir.

In addition, the solid particle flow apparatus has a bypass conduitwhich is connected to the outlet of the separation chamber and which ispositionable to channel solid particles which are larger than thecritical size from the separation chamber to the lower reservoir. Thesolid particles which are channeled through the bypass conduit do notenter the upper reservoir exterior of the separation chamber. Asubstantial portion of the liquid exits the separation chamber throughthe apertures to the upper reservoir exterior of the separation chamber.The liquid is then recombined with the separated solid particles in thelower reservoir after first passing through the heat exchange tubes.

The bypass conduit of the present invention may be positionable tochannel the separated solid particles to one or more of the plurality ofheat exchange tubes for travel to the lower reservoir while preventingthe separated solid particles from entering the remainder of theplurality of heat exchange tubes.

The separation chamber of the present invention, or a portion thereof,may be substantially conical in shape. The separation chamber may beconstructed from steel screen material, or from steel material in whichapertures have been punched or formed.

The separation chamber may be sized to fit between an upper wall of theevaporator and the top of the heat exchange tubes. Further, theseparation chamber may be attached for support to an upper wall of theevaporator. The chamber inlet may be positioned in an upper portion ofthe separation. Similarly, the chamber outlet may be positioned in alower portion of of the chamber. For example, the chamber may be conicalin shape and may have an outlet in a lower portion of theconically-shaped chamber. The chamber may also have tapered sidewallsand may have an outlet positioned at a lower point of the taperedsidewalls.

The present invention also includes a method for selectively andcontinuously separating solid particles having a size larger than acritical size from liquid in a slurry or solution being concentrated inan evaporator. The critical size is a predetermined size above which thesolid particles cause unacceptable clogging in various apertures in theevaporator. The method includes the steps of providing at least a firstreservoir for the slurry or solution; separating from the liquid solidparticles having a size larger than a critical size; channeling to thefirst reservoir the solid particles which were removed from the liquid;channeling to the first reservoir the solid particles which were removedfrom the liquid; and channeling the concentrated liquid to the firstreservoir.

The separation step of the method of the present invention may alsoinclude the steps of providing a separation chamber having apertureswith a size substantially equal to the critical size or smaller;providing a second reservoir in fluid communication with the heatexchanger; positioning the separation chamber within the secondreservoir; and recirculating the slurry or solution from the firstreservoir into the separation chamber. The step of channeling the solidparticles to the first reservoir may further include the step ofproviding a bypass conduit connected to and leading from the separationchamber to the first reservoir without passage of the separated solidparticles through the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a prior art evaporator without the apparatus ofthe present invention.

FIG. 2 shows an evaporator fitted with the apparatus of the presentinvention.

FIG. 3 shows a detailed view of the apparatus of the present invention.

FIG. 4 is a flow chart schematically illustrating the steps of themethod taught by the present invention.

PREFERRED EMBODIMENT

The solid particle flow apparatus of the present invention is designedfor use with an evaporator for concentrating slurries and/or solutions.A typical prior art falling film evaporator is shown in FIG. 1. Suchevaporators usually include a plurality of heat exchange tubes 10arranged substantially vertically. The evaporator also has an upperreservoir or flood box 12, a lower reservoir or sump 14, and arecirculation means 16 for recirculating the slurry or solution from thelower reservoir to the upper reservoir. The recirculation meanstypically includes at least one recirculation conduit 20, and arecirculation pump 18. Also typically present in evaporators, but notshown for clarity, are heating means, means for introducing slurry orsolution to the evaporator, and means for removing concentrated slurryor solution from the evaporator.

In operation, an evaporator such as the one depicted in FIG. 1continuously circulates the slurry or solution from the lower reservoir14, through the recirculating means 16, back to the upper reservoir 12.The slurry or solution repeatedly passes through the heat exchange tubes10, where solvent is evaporated, leaving a more concentrated slurry orsolution.

The heat exchange tubes 10 may be fitted with distributors 22 whichdirect the slurry or solution as it enters the heat exchange tubes. Anexample of distributors is described in U.S. Pat. No. 4,248,296, issuedto Louis J. Jezek and entitled "Fluid Distributor for Condensor Tubes".Distributors, such as those described in the Jezek patent, introduce theslurry or solution into the heat exchange tubes in a film form, thusenhancing evaporation. Such distributors have small apertures which mustremain open in order for the evaporator to run most efficiently. Solidparticles, such as chips or flakes in the slurry or solution can blockand clog the apertures in the distributor, preventing normal thin filmflow and limiting performance of the heat exchange tubes, or ifcompletely blocked, effectively remove the tube from service.

The solid particle flow apparatus of the present invention is designedto prevent such blockage, in distributors or elsewhere in the system, byselectively controlling the flow of solid particles. FIG. 2 shows theevaporator of FIG. 1 fitted with an apparatus of the present invention.The solid particle flow apparatus includes a separation chamber 24 and abypass conduit 26.

FIG. 3 is a more detailed illustration of the solid particle flowapparatus. The separation chamber 24 is fitted within the upperreservoir 12 and is positioned below an outlet 27 of the recirculationconduit 20 to receive the slurry or solution within the interior of theseparation chamber. The separation chamber is preferably attached to acover 28 of the upper reservoir by bolts, screws, pins or other means ofattachment 30.

The separation chamber 24 includes a plurality of apertures 32 which aresized to separate solid particles from liquid. Each chamber aperture 32is of a "critical size." The critical size may vary from system tosystem but should be equal to or smaller than the apertures downstreamof the separation chamber and upstream of the heat exchange tubes. Assuch, the chamber apertures 32 should be of such dimensions as toseparate from the liquid all solid particles which are large enough toblock apertures downstream of the separation chamber and upstream of theheat exchange tubes, for example, the apertures in the distributors 22.

The separation chamber 24 may be constructed from sheet steel into whichholes, preferably round ones, have been punched or otherwise formed. Italso may be constructed from steel screen or mesh material. Preferably,the separation chamber is conical in shape, or at least a lower portion33 of the separation chamber is conical in shape, such as is shown inFIG. 3. This shape helps to funnel the solid particles down to an outlet34 of the separation chamber 24 which is located at the bottom of thelower portion 33 of the separation chamber. The chamber outletcommunicates with the bypass conduit 26. The bypass conduit is attachedto the chamber outlet 34 by conventional attachment means 36. The bypassconduit extends from the separation chamber outlet into at least one ofthe heat exchange tubes 10. Preferably, the bypass conduit is sized tofit within the heat exchange tube. The heat exchange tube into which thebypass conduit is fitted has no distributor in it.

The apparatus of the present invention may be incorporated intoevaporators during original construction, or, may be retrofitted toexisting evaporators. For example, many evaporators have doors, ormanholes, in the top of the upper reservoir 12 to allow operators toenter the upper reservoir to permit its cleaning and maintenance. Theapparatus of the present invention may be constructed outside theevaporator and then installed by an operator entering the upperreservoir through the existing doors. Not only does the presentinvention improve the efficiency of the conventional evaporator, thepresent invention is economical to construct and install as originalequipment or as a retrofit on existing evaporators.

In operation, the slurry or solution circulating in the evaporatorundergoes concentration. It is circulated from the lower reservoir 14 tothe upper reservoir 12 through the recirculation conduit 20. Theseparation chamber 24 receives the recirculated slurry or solution fromthe outlet 27 of the recirculation conduit. Upon entering the separationchamber, the slurry or solution undergoes physical separation, wherebysolid particles which are larger than the critical size are separatedfrom the liquid, and a substantial portion of the liquid component exitsthe separation chamber through the chamber apertures 32. The exitingliquid enters the upper reservoir exterior of the separation chamber andthen enters the heat exchange tubes 10 where it is further concentrated.Solid particles which are larger than the critical size and will notpass through the chamber apertures 32 exit the separation chamberthrough the chamber outlet 34 into the bypass conduit 26. The solidparticles then pass directly to one of the heat exchange tubes 10 intowhich the bypass conduit is fitted for travel to the lower reservoir 14.

In addition to its straining function, the separation chamber 24 alsohelps to reduce the size of the solid particles through turbulence andimpact of the solid particles against the wall of the separationchamber. When the size of the solid particles is sufficiently reduced,the solid particles will no longer cause clogging problems in theevaporator.

The present invention also includes a method for selectively controllingthe flow of solid particles within the evaporator. The followingdiscussion of the method of the present invention is better understoodwith reference to the flow chart shown in FIG. 4.

A slurry or solution undergoing concentration in the evaporator is heldin the reservoir. The slurry or solution has a liquid component andsolid particles, the solid particles usually being of varying sizes. Thefirst step of the method of the present invention is to separate fromthe liquid component the solid particles which are larger than thecritical size. Particles which are larger than the critical size andwhich, therefore, will not flow through downstream apertures will beremoved from the slurry or solution before the slurry or solutioncontinues downstream, i.e., enters the distributors of the evaporator.

A second step in the method is to recirculate the separated solidparticles back to the reservoir containing the slurry or solution.Preferably, this step of recirculation is achieved by washing the solidparticles through a conduit with the aid of some of the liquid from theslurry or solution.

A third step in the method is to direct the separated liquid to the heatexchange tubes of the evaporator. The liquid may contain solidparticles; however, they will all be of a size smaller than the criticalsize and hence will not create blockage problems in the distributors orheat exchange tubes. Within the heat exchange tubes, the liquidundergoes concentration.

A final step in the method is to direct the concentrated liquid back tothe reservoir containing the slurry or solution where it is again mixedwith the solids which were removed in the first step.

The method of the present invention provides for continual circulationof solid particles, and does not provide for a building up of solidparticles at any point in the system, such as would a conventionalfilter. As the solid particles are constantly separated from the liquidand recirculated, at least a portion of them will decrease in size as aresult of the turbulence, and will pose less of a clogging problem.

From the foregoing, it will be appreciated that, although embodiments ofthe invention have been described herein for purposes of illustration,various modifications may be made without deviating from the spirit andscope of the invention. Accordingly, the invention is not limited exceptas by the appended claims.

I claim:
 1. An improved apparatus for selectively controlling the flowof solid particles having a size larger than a critical size for use inan evaporator for concentrating slurries and solutions containing solidparticles and liquid, the critical size being a predetermined size abovewhich the solid particles cause unacceptable clogging of the evaporator,the evaporator having a plurality of generally vertical heat exchangetubes, a lower reservoir positioned below the heat exchange tubes, anupper reservoir positioned above the heat exchange tubes, and arecirculation conduit for recirculating slurry or solution from thelower reservoir to the upper reservoir, wherein the improvementcomprises:a separation chamber having an inlet and an outlet and havinga plurality of apertures sized to separate from the liquid solidparticles larger than the critical size, said separation chamberpositioned within the upper reservoir, with said chamber inletpositioned to receive slurry or solution from the recirculation conduitwithin the interior of said separation chamber, and said chamberapertures communicating a substantial portion of the liquid to the upperreservoir exterior of said separation chamber for passage through theheat exchange tubes to the lower reservoir, said separation chamberbeing sized to fit between an upper wall of the evaporator and the heatexchange tubes, said separation chamber being attached for support tosaid upper wall of the evaporator; and a bypass conduit connected tosaid chamber outlet and positioned to channel from the interior of saidseparation chamber solid particles which are larger than the criticalsize to the lower reservoir without entering the upper reservoirexterior of said separation chamber, with said substantial portion ofthe liquid exiting the interior of said separation chamber through saidchamber apertures to the upper reservoir exterior of said separationchamber being recombined with the separated solid particles in the lowerreservoir after first passing through the heat exchange tubes.
 2. Theapparatus of claim 1 wherein said chamber outlet is positioned atsubstantially the lowest portion of said separation chamber to encouragetravel of the separated solid particles toward said chamber outlet underthe influence of gravity.
 3. The apparatus of claim 1 wherein saidbypass conduit is positioned to channel the solid particles which arelarger than the critical size to at least one of the plurality of heatexchange tubes for travel to the lower reservoir while preventing thesolid particles from entering the remainder of the plurality of heatexchange tubes. lower reservoir after first passing through the heatexchange tubes.
 4. The apparatus of claim 1 wherein said separationchamber is substantially conical in shape.
 5. The apparatus of claim 1wherein a lower portion of the separation chamber is substantiallyconical in shape.
 6. The apparatus of claim 1 wherein said separationchamber is constructed from steel screen material.
 7. The apparatus ofclaim 1 wherein said separation chamber has walls constructed from steelmaterial and said apertures are round holes formed in said steel walls.8. The apparatus of claim 1 wherein said separation chamber inlet ispositioned at an upper portion of said chamber.
 9. The apparatus ofclaim 1 wherein said separation chamber outlet is positioned at a lowerportion of said chamber.
 10. The apparatus of claim 1 wherein saidseparation chamber has tapered sidewalls.
 11. The apparatus of claim 1wherein said separation chamber outlet is positioned at a lower point ofsaid tapered sidewalls.